Method of adapting the air interface in a mobile radio system and corresponding base transceiver station, mobile station and transmission mode

ABSTRACT

A mobile radio system enables exchange of bidirectional digital signals between at least one mobile station and at least one base transceiver station and provides at least two coding modes, each mode corresponding to a predetermined source code and a predetermined channel code for the transmission of a wanted signal for each transmission direction. Each source code corresponds to a given user bit rate and each channel code corresponds to a given coding efficiency and therefore to a given total bit rate for a given source code. For adaptation of the air interface of this system, at the time of a call between a mobile station and a base transceiver station, two separate analyses of transmission quality are carried out for each transmission direction, respectively, and, for each transmission direction, one of the coding modes is selected in accordance with the corresponding transmission quality analysis.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of digital mobile radio systems. Theinvention applies in particular to cellular mobile radio systems such assystems conforming to the GSM (Global System for Mobile communications)standard, for example.

More particularly still, the invention concerns the exchange of digitalsignals, whether they constitute data or speech, in time-divisionmultiple access (TDMA) time-division multiplex systems.

2. Description of the Prior Art

The TDMA technique divides time into frames of fixed and predeterminedduration, the frames being in turn divided into time slots. Each call isassociated with one or more time slots.

Thus a frame comprises N time slots that can correspond to N calls. Eachreceiver is able to extract the time slots addressed to it in order toreconstitute the source signal. In this way N calls can be transmittedin the same frequency band.

Where data communications are concerned, mobile radio systems like theGSM system conventionally provide two services, corresponding to twodifferent, levels of quality. Thus, for transmission of data, the GSMsystem provides a first data communication mode called the full ratemode, in which a time slot is transmitted in each frame, and a secondtransmission mode called the half rate mode, in which the data signal istransmitted in one time slot only every two frames, on average.

In this second mode, the resource allocated to a call is halved comparedto the first mode. This frees up resources for other calls.

This halving of the total bit rate of the call naturally requiresmodification of the channel coding used, to retain the same user bitrate, in other words, the half rate mode corresponds to channel codingwith half the yield of that of the full rate mode. In the GSM system,the two modes respectively correspond to raw bit rates of 11.4 kbit/sand 22.8 kbit/s.

Consequently, the efficacy of half rate channel coding is less than thatof full rate channel coding. For this reason the half rate mode can beused only when transmission conditions are good and/or the transmissionquality required is average, in other words when a relatively high biterror rate can be tolerated. If the transmission channel is subject tointerference and/or the data requires a higher transmission quality(i.e. a lower bit error rate) the full rate mode must be used.

According to the GSM standard, a transmission mode is chosen at the timethe call is set up and is retained throughout the call. This techniquehas two drawbacks:

if the service in question requires a transmission quality correspondingto the use of the half rate mode under normal conditions of operabilityand if the half rate mode is adopted, should the channel then besubjected to a higher level of interference, exceeding the operabilitylimit of the system (set at a C/I value of approximately 9 dB), the callin progress is suddenly cut off; under difficult coverage conditions avalue of C/I below 9 dB may be encountered;

if the service in question requires a transmission quality correspondingto the use of the full rate mode under normal conditions of operabilityand if the full rate mode is adopted, should the channel subsequently besubject to a lower degree of interference, the channel coding employedis of higher quality than is required; the transmission channel istherefore unnecessarily occupied in alternate frames (causingunnecessary interference in neighboring cells).

In mobile radio systems these problems are major problems since thetransmission channel changes continually with the movement of the mobilestation and the movement and the activity of the sources ofinterference, etc. As a result, the full rate mode is usually chosen,for safety, and this leads to high and often unnecessary consumption ofthe transmission resource.

There are also two configurations in the case of speech signals (fullrate mode and half rate mode), which correspond to the use of differentspeech encoders (source coding) and different channel encoders, the twopairs of encoders (source and channel) providing respective raw bitrates of 22.8 kbit/s (full rate) and 11.4 kbit/s (half rate).

Problems similar to those described above for data are also encounteredwith speech.

A major objective of mobile radio system designers is to limit thequantity of data transmitted, for a number of reasons and in particular:

to increase the number of calls in the multiplex;

to reduce the transmission time (in the case of transmitting data);

. .

To this end, U.S. patent application U.S. Pat. No. 5,327,576 proposesmodification of the mode of transmission used, during a call, on thebasis of the measured bit error rate.

To be more precise, in the method described in the above application,the base transceiver station (the station managing all calls withmobiles in a given cell) measures the bit error rate of a given call andselects one or other of the transmission modes according to the measurederror rate.

This technique improves transmission resource use. It has a number ofdrawbacks, however, that the novel technical approach of the inventionclearly highlights. In particular, it is based on an analysis of thetransmission channel as seen from the base transceiver station only (orfrom the mobile station only), which leads to non-optimum resource use,as will emerge below.

One object of the invention is to overcome these various drawbacks ofthe prior art.

To be more precise, one object of the invention is to provide a methodof adaptation of the air interface (essentially corresponding to layers1 (physical) and 2 (link) of the ISO model) in a mobile radio systemthat minimizes the occupancy of transmission channels by reducing thequantity of resource allocated to a call on average and by limitinginterference induced by a call in neighboring cells.

This object of limiting interference is crucial in cellular mobile radiosystems in particular. In these systems, the same frequency band isallocated to several geographically dispersed cells. Although thedistribution of the cells is defined to maximize the distance betweenthem, it is by no means rare for the signals of a given cell to sufferinterference from those of other cells using the same band to a degreethat is above an acceptable limit for the system.

In a cellular system, a maximum interference level enabling thespecified transmission quality to be provided is usually fixed. Anobject of the invention is therefore to provide a method of the abovekind whereby the specified transmission quality continues to be providedif the interference exceeds this maximum level.

An object of the invention is therefore to provide a method of the abovekind in which the untimely cutting off of calls is reduced.

In other words, an object of the invention is to expand the range ofoperability of the system, in particular under difficult transmissionconditions.

Another essential and primordial object of the invention is to provide amethod of the above kind whereby the number of calls can be increased.In other words, an object of the invention is to reduce, on average, theresource used to transmit a service in order to increase the number ofusers in the system, i.e. the number of calls per cell.

In one particular embodiment of the invention, a secondary object of theinvention is to provide a method of the above kind for transmittingasynchronous data easily and as fast as possible, in particular when itis not possible to free up the same resource in both communicationdirections.

Another object of the invention is to provide a method whereby thenetwork infrastructure is simplified. In the conventional way, networkplanning must offer acceptable operability (C/I≈9 dB in the GSM system)everywhere (or over a certain portion of the area of each cell), whichin some cases constitutes a very serious constraint.

An object of the invention is therefore to remove this constraint bywidening the operability range (beyond 9 dB, for example in the case ofthe GSM system) to allow more efficient planning by reducing the numberof sites.

Another object of the invention is to provide a method of the above kindthat is equally applicable to speech signals and to data signals.

SUMMARY OF THE INVENTION

These objects, and others that will emerge hereinafter, are achieved inaccordance with the invention by means of a method of adaptation of theair interface in a mobile radio system enabling exchange ofbidirectional digital signals between at least one mobile station and atleast one network entity such as a base transceiver station, andproviding at least two coding modes, each mode corresponding to apredetermined source code and a predetermined channel code for thetransmission of a wanted signal for each transmission direction, andeach source code corresponding to a given user bit rate and each channelcode corresponding to a given coding efficiency and therefore to a giventotal bit rate for a given source code, wherein, at the time of a callbetween a mobile station and a base transceiver station, two separateanalyses of transmission quality are carried out for each transmissiondirection, respectively, and, for each transmission direction, one ofsaid coding modes is selected in accordance with the correspondingtransmission quality analysis.

The invention is based on a novel approach to the transmission channel,which is treated separately for each transmission direction.Transmission conditions can be very different for the two transmissiondirections. One reason for this is that the sources of interference aredifferent. In the uplink direction, from the mobile station to the basetransceiver station, the sources of interference are essentially themobile stations moving in the co-channel cells; in the opposite,downlink direction, the sources of interference are neighboring basetransceiver stations. The masking effects are therefore different.

The invention therefore optimizes three things:

the analysis of transmission conditions, which were previously viewedfrom one end only (generally the base transceiver station); it wastherefore possible to regard as "good" a channel that was "bad" in theopposite direction, and vice versa; the invention presupposes feedbackof quality information (in at least one direction) before a decision istaken;

the selection of the coding mode, which is selective for eachtransmission direction; and

the encoding mode used, since the invention enables selective action atthe source coding and/or channel coding level.

It should be noted that the approach adopted by the invention is in noway obvious in the light of the prior art techniques. Apart from thefact that it is based on a novel approach to the transmission channel, anumber of modifications are required in order to put the invention intoeffect, in particular with regard to the exchange of information(measured quality and/or change of coding mode) between the twostations.

Some mobile radio systems, like the GSM system, allocate resources on afixed basis, through at least two transmission modes corresponding todifferent transmission resources allocated to a call.

To be more precise, a transmission mode defines a coding mode/allocatedresource combination. The transmission mode therefore corresponds to theuse of a certain coding mode and the allocation of a certain resource. Acoding mode can therefore correspond to a plurality of transmissionmodes.

In this case, in accordance with the invention, on the occasion of acall between a mobile station and a base transceiver station, one ofsaid unidirectional transmission modes is advantageously selected foreach transmission direction, uplink and downlink, in accordance with atleast one of said transmission quality analyses and the quality requiredfor said call, and possibly the traffic load.

In other words, a transmission mode is chosen when a call isinitialized, corresponding to a required level of quality, for example.The invention enables the transmission mode to be changed during a call,corresponding to a change of coding mode, as soon as this is possible(changing to a mode that is less robust with respect to transmissionerrors, but offering a higher level of quality and/or a lowerconsumption of resources) or necessary (changing to a mode that is morerobust with respect to transmission errors, but consuming more resourcesand/or enabling the service to be maintained at the cost of a slightreduction in quality, subject to constraints of availability, ofcourse).

Various situations can be envisaged, according to whether the mobileradio system in question requires symmetrical allocation of resourcesfor each transmission direction or not.

In the second case, which can be regarded as equivalent to twounidirectional calls, the principle can be very simple: the change ofmode in one direction is effected as soon as the change of coding modeto a less robust, respectively more robust mode with respect totransmission errors is possible, respectively necessary. A change of thecoding mode is necessary if the quality of the channel deteriorates. Itis possible if the quality of the channel improves to the point thattransmission quality can be maintained using a coding mode consumingless resource or increased if the resource already allocated isretained.

In the former case, which corresponds to the GSM system, for example,direct application always leads to the choice of identical coding modesfor both directions. There is therefore a bidirectional transmissionmode that corresponds to the use of the same unidirectional mode in bothtransmission directions. A change of bidirectional transmission mode istherefore effected if a change of coding mode (compatible with thetransmission mode) is possible for both directions or necessary for atleast one direction.

Thus, in a system in which said transmission modes correspond to anallocation of identical transmission resources in each of saidtransmission directions, a change of transmission mode to a transmissionmode corresponding to a greater transmission resource is effected if atleast one of the coding modes selected in at least one transmissiondirection corresponds to a total bit rate incompatible with the resourceallocated in the current transmission mode and the additionaltransmission resource required is available (in which case the samecoding mode is applied to both transmission directions, this coding modebeing that of the two modes selected requiring the greater amount of theresource), and a change of transmission mode to a transmission modecorresponding to a lesser amount of transmission resourceis effected ifcoding modes consuming less resource are selected in both directions. Abidirectional transmission mode is then chosen corresponding to the useof the same coding mode in both directions, the latter mode being thatof the two modes selected in each direction consuming the greater amountof the resource.

The same approach can be adopted for selecting between bidirectionaltransmission modes which are the same in terms of use of resource butdifferent in terms of robustness.

The invention is naturally not limited to two coding modes or to twotransmission modes. To the contrary, it is a simple matter to generalizethe invention to n coding modes and m unidirectional transmission modes(m≦n, it being possible for the same coding mode to correspond to aplurality of transmission modes). On the basis of n coding modes it ispossible to define n bidirectional transmission modes for which the samecoding mode is used in both directions.

In accordance with one important feature of the invention, at least onemodified transmission mode is also defined, in which the coding modesused in the two directions are different.

Accordingly, in the situation in which there are n differentunidirectional coding modes, it is possible to define:

n bidirectional transmission modes (known as primary modes)corresponding to the use of the same coding mode in both directions;

n(n-1) bidirectional transmission modes (known as secondary modes)corresponding to coding modes in both directions. Among these n(n-1)modes there may exist situations in which the raw bit rate is differentand therefore corresponds to different required minimal resources in thetwo transmission directions (to be more precise, there are modifiedtransmission modes of two types: modes with the same raw bit rate inboth directions, and modes with asymmetric raw bit rates). If the systemrequires symmetrical resource allocation, the resource corresponding tothe highest raw bit rate is allocated. The resource allocated to atleast one of said transmission directions is therefore greater than theresource required to transmit the information coded in the correspondingcoding mode, and said coded information is divided between a fraction ofthe time slots corresponding to said allocated resource.

This type of modified transmission mode is entirely novel. It ispossible only because the invention provides a different approach toquality for each transmission direction. In the case of a bidirectionalmode with asymmetric raw bit rates, it enables time slots to be freed upin one direction even if this is not possible in the other direction.Moreover, the invention is equally specifically concerned with amodified transmission mode of this kind.

Two strategies can be envisaged for time slots that are not used by saidcall:

either they do not carry any signal, which reduces interference withneighboring cells; if there is no transmission in a cell, the latterdoes not cause any interference in its neighbors,

or they are allocated to the transmission of asynchronous data.

In one particular embodiment of the invention (intended in particularfor adaptation of the current GSM standard for data communications),said transmission modes comprise:

a first (full rate) mode in which said data is transmitted at the rateof one time slot every signal frame, and

a second (half rate) mode in which data is transmitted at the rate ofone time slot every two signal frames.

In this case, a modified transmission mode advantageously consists in:

transmitting information coded in a first coding mode at the rate of onetime slot every signal frame (full rate) in a first transmissiondirection, and

transmitting information coded in a second coding mode at the rate ofone time slot every two signal frames (half rate) in a secondtransmission direction,

the resources allocated to the call in both transmission directionscorresponding to the resource needed to transmit data in said firstmode.

The analysis of transmission quality advantageously consists indetermining at least one of the following:

the bit error rate (BER) of the received signal,

the power of the received signal,

the distance between the mobile station and the base transceiverstation,

an estimate of the impulse response of the transmission channel,

the time alignment,

the signal to noise ratio,

the signal to interference ratio (C/I).

In one advantageous embodiment of the invention said selection of acoding mode allows additionally for at least one of the following:

a required level of quality for the call in progress,

a required level of quality for at least one transmission direction andfor the call in progress,

a type of service conveyed by said call,

the traffic load.

The selection of a coding mode preferably includes a step of comparinginformation representative of the transmission quality with at least onepredetermined threshold, to be more precise with the same number ofthresholds as coding modes.

Said quality information is advantageously compared with differentthresholds according to the level of quality required for the call inprogress, if there is more than one level of quality.

It is advantageous to define two sets each of at least one threshold, afirst set being used when the measured transmission quality deterioratesand a second set being used when the measured transmission qualityimproves.

This avoids incessant changing of modes when the measured level is neara threshold (this is known as the "ping-pong" effect).

Said thresholds are preferably predetermined values of the signal tointerference ratio (C/I).

In a preferred embodiment the decision to change coding mode and/ortransmission mode is taken in said base transceiver station, said mobilestation transmitting to said base transceiver station informationrepresentative of transmission quality in the base transceiver stationto mobile station direction.

More generally, the method of the invention preferably includes a stepof selecting between at least two source codes and/or a step ofselecting between at least two channel codes.

Said selection of a coding mode is carried out in such manner as tolimit the quantity of resource allocated in each transmission directionand/or to optimize transmission quality.

For example, a source code and a channel code may be chosen to maintainthe current raw bit rate as far as possible, and therefore to offer thebest possible transmission quality without modification of the resource,or to provide the best possible transmission quality subject tomodification of the resource.

The invention also concerns a base transceiver station of a mobile radiosystem implementing the method as defined above. A base transceiverstation of this kind advantageously comprises:

means for determining at least a first indication representative oftransmission quality in the mobile station to base transceiver stationdirection,

means for receiving a second indication representative of transmissionquality in the base transceiver station to mobile station direction,

means for modifying the coding mode and/or the transmission mode in eachtransmission direction in accordance with said first and secondindications, and

means for transmitting to said mobile station information representativeof the coding and/or transmission modes selected.

It also concerns a corresponding mobile station including:

means for determining at least one indication representative oftransmission quality in the base transceiver station to mobile stationdirection,

means for transmitting said indication to said base transceiver station,and

means for receiving an indication representative of the coding and/ortransmission modes selected.

Other features and advantages of the invention will emerge from areading of the following description of a preferred embodiment of theinvention given by way of non-limiting illustrative example and from theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a type of cellular mobileradio network that is known in itself and in which the method of theinvention may be used.

FIG. 2 shows a first embodiment of the invention in the case of a doubleunidirectional call, each transmission direction being managedindependently.

FIGS. 3A through 3C show the three modes of data transmission used anadvantageous embodiment of the invention, respectively corresponding tothe full rate and half rate modes that are known in themselves and thenew mode of the invention, known as the modified half rate mode.

FIGS. 4A and 4B show the principle of the decision to change modeaccording to the required level of quality for the call in progress andFIGS. 5A and 5B show the various corresponding operationalpossibilities.

FIG. 6 shows the variation in the bit error rate as a function of theratio C/I, according to the required level of quality, and one possibledetermination of the threshold values from FIGS. 4A, 4B, 5A and 5B.

FIG. 7 shows a block diagram of the transmit part of a base transceiverstation of the invention.

FIG. 8 shows the decision process used by the station from FIG. 7 in thecase of n=2 modes.

FIG. 9 is a block diagram showing the general principle of theinvention.

FIG. 10 shows one embodiment of the method of the invention generalizingthe FIG. 2 embodiment to the selection of the coding mode from n codingmodes.

FIG. 11 shows the various possible bidirectional transmission modes whenn codes are available in each transmission direction, in accordance withquality information.

FIG. 12 is a generalization of FIG. 6 to the case of n coding modescorresponding to n channel codes for the data at constant user bit rate.

FIG. 13 shows graphs for the MOS performance of the various coding modesfor speech (combination of two speech encoders and three channelencoders).

FIG. 14 shows in the form of a matrix one way of numbering thetransmission modes of a system with n coding modes in each direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a diagrammatic representation of a type of cellular networkthat is known in itself. The geographical territory covered by themobile radio system is divided into cells 111 through 11N. Each cell 111includes a base transceiver station 121 that can exchange signals 131,132 with a plurality of mobile stations 141, 142 moving within said cell111.

Two neighboring cells 111, 112 use different frequency bands so thatthere is no interference between the signals transmitted in the twocells. To be more precise, the allocation of frequencies is based on anorganization of the cells into patterns 151, 152 each of seven cells. Ofcourse, the patterns (and likewise the cells) can be different shapesand the patterns can contain more than seven cells. The frequenciesallocated within the same pattern are different. On the other hand, theyare re-used from one pattern 151 to the other 152, for example, cell 115uses the same frequencies as cell 111.

Consequently, the signals 131, 132 exchanged in the cell 111 may causeinterference 16 with the signals exchanged in the cell 115. One objectof the invention is to limit this interference (another essentialobjective being, of course, to increase the number of calls possible ineach cell) by minimizing the exchanges 131, 132. If no signals 131 or132 are transmitted, there is naturally no interference 16.

To achieve this, the main feature of the invention is control of thecoding mode on the basis of a two-fold analysis of the transmissionchannel, the latter being considered independently for each transmissiondirection. In this way it is possible to limit the signals transmittedselectively for each transmission direction.

Taking another approach, this techniques makes it possible to free upresources to transmit more data or speech, and also to optimizetransmission quality.

FIG. 9 shows the main features of the method of the invention. In FIG.9, reference numbers relating to the uplink transmission direction havethe suffix M and those concerning the downlink direction have the suffixD.

As clearly shown in this figure, coding is optimized independently foreach transmission direction. To this end, two separate qualitymeasurements 91M and 91D are effected for each transmission direction(as already indicated, transmission quality can be very different fromone direction to the other).

Then, for each direction, a coding mode 93M, 93D is selected (92M, 92D)in accordance with the measured quality indicator 91M, 91D, a trafficload indicator 925M, 925D and, where applicable, a required transmissionquality 94M, 94D.

The coding mode is chosen (92M, 92D) in two stages:

a source code is chosen (921M, 921D),

a channel code is chosen (922M, 922D).

In the case of data, the expression "source code" naturally correspondsto the user bit rate concept.

The above two-fold choice is made, for example, to limit the bit rateand to optimize coding quality, in accordance with the characteristicsof the transmission channel.

The two choices are naturally not independent. To the contrary, a codingmode is chosen that corresponds to a user bit rate (source coding) and agiven channel code, as shown by the arrows 923M, 923D, 924M, 924D.

The coding modes may be chosen in either station. In all cases, eachstation measures the quality of the received signal one or more timesand then chooses the coding mode or transmits the measurement(s) to theother station so that the latter can make the corresponding choice.

The method further includes an additional step 95 of transmissionresource allocation.

The allocation 95 is made in such manner as to enable the transmissionof information coded in the coding mode 93M, 93D selected on the basisof the availability indicators 96M, 96D, of course, and again with theobjective of limiting the resource used.

If the system requires symmetrical resource allocation, it allows forthe coding mode 93M, 93D having the highest raw bit rate.

The choice of coding mode is then transmitted to the other station sothat it can adapt its coding and/or its decoding accordingly.

The method of the invention can also take into account trafficinformation 925M and 925D in choosing the coding mode and/or thetransmission mode, especially if the user bit rate is variable (inparticular for data services).

The coding and transmission mode(s) 97 selected are then transmitted(98) to the other station, so that the latter can adapt its codingand/or its decoding accordingly.

Various embodiments of the invention can therefore be envisaged,depending on the type of system concerned. FIG. 2 shows a firstembodiment of the invention in which each transmission direction ismanaged independently (double unidirectional call) and in which twotransmission modes are available for a constant user bit rate.

This is the most simple case since there is no particular problem withresource asymmetry. FIG. 2 shows the processing carried out in themaster station (base transceiver station or mobile station) if the callconcerns exchange of data.

In each station (base transceiver station and mobile station) atransmitted signal is received (21) and then its quality Q is determined(22) by a method that is known in itself. There are many indicators ofsignal quality, some of which are already calculated in the stations forother purposes. They include the following criteria:

the bit error rate (BER) of the received signal,

the power of the received signal,

the distance between the mobile station and the base transceiverstation,

an estimate of the impulse response of the transmission channel,

the time alignment,

the signal to noise ratio,

the C/I ratio.

More than one of these criteria can naturally be considered in order torefine the analysis.

The mobile station transmits (29) the quality information to the basetransceiver station which takes the decisions for each transmissiondirection.

The system provides at least two transmission modes, for example thefull rate (FR) mode and the half rate (HR) mode, corresponding todifferent channel coding qualities (coding efficiency). It is possibleto change mode during transmission for each transmission direction,according to the value IQ.

The following comparisons are therefore made, depending on the currenttransmission mode (23):

if the transmission mode is the full rate mode 24 and the quality IQ isabove a predefined threshold S[1] (25), it is possible to change to thehalf rate mode (210); this limits the bit rate whilst assuringsufficient transmission quality;

if the transmission mode is the half rate mode 26 and the quality IQ isabove a threshold S[2] (27), it is desirable to change (211) to the fullrate mode to maintain reception quality.

In the case of a speech signal, the two modes considered can be the halfrate mode (in the sense defined in the GSM standard) and theoverprotected half rate mode (corresponding to the use of a speechencoder corresponding to the half rate mode with channel encoding mademore robust, so that the resource occupied is one time slot in everyframe). A single threshold is then used (there are no multiple qualitylevels).

If the outcome of one of the tests 25 and 26 is positive, a change modecommand is sent (28) to the mobile station and the resource is modifiedaccordingly. Otherwise, no change is effected.

The transmission 28 of a command can of course take into account manyother criteria and in particular the available resource. It can alsoincorporate a time-delay to avoid incessant changes.

It is advantageous to provide the facility to require a given qualitylevel (from at least two candidates) at initialization of the call,possibly different for the two transmission directions. In this case, asmany thresholds Qi as necessary are defined.

Thus FIG. 10 shows the generalization of the FIG. 2 situation to nunidirectional modes (for constant user bit rate data services orspeech).

After the call is created (101) with a quality level i chosen from npossible levels, a quality indicator IQ is determined regularly (102),or obtained from the other station, after which IQ is compared (103) toa series of thresholds Si. For example, if the quality criterion is C/I,IQ is compared to C/I[i] and the coding mode k is selected such that:

    C/I[k]≦IQ≦C/I[k-1]

If the coding mode k is that already selected (104) no processing isdone (105). Otherwise, the required change of coding mode is effected(106), after which the change of mode information is transmitted (107)to the other station.

Note that the quality information IQ may be expressed in terms otherthan the transmission quality of the service. For data, for example, itmay be expressed in terms of the bit error rate (BER) and the decisioncan be based on the distance.

Of course, the order of the thresholds may be reversed, for example ifthe criterion chosen is the distance. For a relatively long distance,quality is statistically worse and a more robust coding mode is needed.For a relatively short distance, on the other hand, the C/I ratio isstatistically greater, and is characteristic of a favorable situation.

In the case of data services for which the user bit rate may vary, it isnecessary to accept a variable information transmission time-delay. Onthe other hand, the information must be received with at least a minimumquality. There may therefore be a series of modes corresponding to avariable user bit rate with a constant or variable raw bit rate, thelatter implying a variable resource. This introduces differenttransmission times for the same quantity of information transmitted.

It is possible to change mode on the basis of the channel quality butalso on the basis of the traffic load of the network. For each user bitrate there will be a series of modes corresponding to a differentresource. This amounts to adding a dimension for the classification ofthe modes. The mode assuring the required quality will be selected fromthe coding modes leading to the use of a particular resource inaccordance with the traffic load.

The traffic load may be expressed in terms of the number of time slotsused in the cell, which is known to the base transceiver station.

In a variant of this method, it is possible to transmit the qualityinformation systematically, the receiver then taking the decision tochange modes for itself. This technique simplifies the verification ofresource availability. On the other hand, it requires the transmissionof information indicating the mode used.

In many mobile radio systems, and in the GSM system in particular,resource allocation is symmetrical: the same quantity of resource mustbe allocated to each transmission direction.

In this case, the transmission mode change (in the usual sense of thisexpression) is possible only when it is acceptable or necessary in bothtransmission directions. However, the invention proposes a new mode, fordata, called the modified mode, which is based on a novel, two-levelapproach: coding changes in each transmission direction and modechanges.

In the case of speech, a similar approach can be adopted, substituting"overprotected half rate" for all references to "full rate".

FIGS. 3A through 3C show the three possible modes in the situation inwhich two bidirectional transmission modes (full rate and half rate) areavailable for a data service at a constant user bit rate. Various othermodes on the same model can naturally be defined.

FIG. 3A shows the full rate mode known in itself in which a time slot31i, 32i is associated with each frame 33i, 34i in each transmissiondirection.

In this transmission mode the data is coded using a first channel codethat corresponds to a bit rate of 22.8 kbit/s (in the GSM system).

FIG. 3B shows the second mode that is known in itself, the half ratemode, corresponding to a second channel code and a bit rate of 11.4kbit/s. The first and second codes can correspond to two different errorcorrecting code rates, for example.

In this second mode, a time slot 35i, 36i is allocated to the call onlyin one frame in two 371, 373, 381, 383. In the intermediate frames 372,382, the corresponding time slot 39, 310 can be allocated to anothercall.

FIG. 3C shows the modified half rate mode of the invention. Thismodified mode is used if the transmission channel allows the use ofdifferent channel codes in the two transmission directions. In thiscase, the same resource is allocated in the full rate mode (FIG. 3A):each frame 311i, 312i includes a time slot 313i, 314i for the call, but:

in a first direction, the first channel code is used and the time slots313i carry data,

in a second direction, the second channel code is used, and correspondsto half the bit rate of the previous code, and data is transmitted onlyin one time slot 3141, 3143 in two; the slots 3142 that are not used areeither allocated to the transmission of asynchronous data or left emptyto limit interference.

A system of this type that is known in itself therefore allows two typesof service to be defined, corresponding to different qualities. Thequality is expressed in terms of the output bit error rate (BER), forexample. A service corresponds to the use of one transmission mode inboth directions under conditions of operability (value of C/I or powerlevel received Eb/No). In the GSM system, for example, the operabilitylimit corresponds to C/I=9 dB.

The service implemented therefore belongs to a set {"high" qualityservice, "average" quality service}, respectively corresponding to ahigh bit rate and to a lower bit rate under normal conditions.

In accordance with the invention, it is possible to modify thetransmission mode during a call with the two-fold objective ofmaintaining the required level of quality and limiting the quantity ofinformation transmitted.

FIGS. 4A and 4B show the decision principle in the respective two typesof quality, and therefore of service, and FIGS. 5A and 5B show thevarious operational possibilities.

FIG. 4A corresponds to the high quality level. If C/I is between S[2](=9 dB) and S[1] (>9 dB) the full rate coding mode 41 is used. BeyondS[1] the half rate coding mode 42 is used. Below S[2] the call may beinterrupted.

This makes it possible to distinguish between the four types offunctioning shown in FIG. 5A, for a bidirectional call, when a highquality level is required:

51: C/I between 9 dB and S[1] in both directions: full rate mode in bothdirections,

52 and 53: C/I between 9 dB and S[1] in one direction and greater thanS[1] in the other direction: full rate mode in one direction andmodified half rate mode of the invention in the other direction,

54: C/I greater than S[1] in both directions: modified half rate mode inboth directions, if the system does not provide for changing theallocation of resources during a call, or half rate mode in bothdirections if this change is possible.

FIG. 4B corresponds to the lower quality level. If C/I is between S[1]=9 dB and S[2], the full rate coding mode 43 is used. Beyond S[2] halfrate coding 44 is used. Below S[2] the call may be interrupted.

This makes it possible to distinguish between the four types offunctioning shown in FIG. 5B, when a high quality level is required:

55: C/I between S[2] and S[1] =9 dB in both directions: full rate modein both directions,

56 and 57: C/I between S[2] and S[1] =9 dB in one direction and greaterthan S[1] =9 dB in the other direction: full rate mode in one directionand modified half rate mode of the invention in the other direction,

58: C/I greater than S[1] =9 dB in both directions: modified half ratemode in both directions, if the system does not provide for changing theallocation of resources during a call, or half rate mode in bothdirections if this change is possible.

FIG. 6 shows the variation in the bit error rate BER as a function ofC/I for the full rate mode 61 and for the half rate or modified halfrate mode 62.

C/I is the normal operability limit (C/I =9 dB, for example). Therequired quality levels Q1, Q2, respectively, define the thresholds S[1]and S[2] (where S[i] is a threshold for deciding to change from codingmode i to i+1 (i.e. half rate (i =1) to full rate (i =2)) and viceversa, for both types of service, of medium and high quality,respectively, equivalent to using the half rate and full rate modes,respectively, under normal operability conditions.

In the example described above the coding rates are in a ratio of 2:1.Any other ratio may naturally be chosen. Moreover, this principle caneasily be generalized to a greater number n of coding modes.

Consider the case of n different coding modes, numbered from 1 throughn, in order of increasing robustness, and therefore in order of possiblyincreasing allocated resource, and still assuming that the resourceallocated is bidirectional: there are n² (n+(n(n-1))) bidirectionaltransmission modes. The first n modes correspond to the use of the samecoding mode i in both directions. The (n(n-1)) other bidirectional modescorrespond to the use of different coding modes.

In this latter case, the resource allocated is that required by the mostdemanding coding mode. Accordingly, if Ci denotes the codes and Mi themodes:

for 1≦i≦n: Mi corresponds to the code Ci in both directions;

for n+1≦i≦n² : Mi corresponds to the code Cl in one direction and thecode Cm in the other direction, where l and m are obtained from thesymmetrical matrix shown in FIG. 14.

The columns 1411 through 141n correspond to the selected coding mode inthe uplink direction. The rows 1421 through 142n correspond to thecoding mode in the downlink direction. This defines a notation for then(n-1) transmission modes 143i.

The quality Qi is expressed in terms of the bit error rate for dataservices, for example. Qi can be expressed in terms of the Mean OpinionScore (MOS) for speech transmission services.

There are therefore at most n different services. There can be up to ndata services, but for speech there is only one service and the aim isto extend the range of operability.

FIG. 12 is similar to FIG. 6 but shows n curves 1211 through 121ncorresponding to the n codes in the case of fixed user bit rate data.

The thresholds C/I[i,j] are defined as minimal values of C/I enabling acode Cj to assure the quality Qi. In the present context, C/I[i,j]corresponds to the abscissa of the point on the curve giving theperformance of the code i for an ordinate equal to Qi. For a so-callednormal operability condition, C/I[i,i]=operability point =9 dB in theGSM system.

For speech transmission, in terms of MOS, the only difference is thatthe curves are inverted. This is because better quality corresponds to abetter MOS.

FIG. 11 shows the various possibilities in this case (FIG. 11 is anextension of FIGS. 5A and 5B), for a given service i. The thresholdsS[j] then correspond to C/I[i,j]. The robustness of the code increasesfor increasing values of n.

The n different thresholds S[i] to be considered are plotted on theabscissae and on the ordinates. The thresholds define areas 111k,l inwhich the combination (Ck, Cl) is indicated, where Ck is the coding modeto use in the uplink direction and Cl is the coding mode to use in thedownlink direction.

If k=l, the transmission mode k is selected (in the situation in whichthe system requires symmetrical allocation); otherwise, the FIG. 14matrix is used to determine the bidirectional mode.

Below the threshold S[n] no call can be made (112).

The decisions to change mode are advantageously taken in only one of thetwo stations communicating, preferably the base transceiver station.These decisions can of course be taken in the mobile station, however.

FIG. 7 shows the means employed in a base transceiver station for n=2.The mode control means 71 determine the type of channel code to be usedin the base transceiver station to mobile station direction, operatingon the selector 72 by means of the command 73 to choose a channelencoder from the channel encoders 7131 through 713N, and in the mobilestation to base transceiver station direction, by transmitting a changemode command 74 to the mobile station if necessary.

The control means 71 operate on the basis of a required qualityindication 75, information 76 representative of the transmission qualityin the mobile station to base transceiver station direction, obtained byanalysis 77 of the received signal 78, and information 79 representativeof the transmission quality in the base transceiver station to mobilestation direction, extracted (710) from the data transmitted by themobile station.

The processing carried out by the module 71 is described in more detailbelow, with reference to FIG. 8.

It is clear that the same principle can be used to select a source codefrom a plurality of source codes, for example using the command 723.

The data 711 to be transmitted to the mobile station undergoes sourcecoding 712 and then channel coding 713i, according to the position ofthe selector 72.

The codes 713i correspond to different coding rates, for example. If thecoding mode selected for the uplink and downlink directions givedifferent raw bit rates (which is automatically the case if a singlespeech encoder is used and if two different channel codes are employed),a so-called modified bidirectional transmission mode is used and thedata coded for one transmission direction uses only part of theallocated resource. The base transceiver station can then authorize thetransmission of asynchronous data in the time slots left free.

In the case of a system with two channel encoders corresponding to thetransmission modes of FIGS. 3A and 3B, for example, the base transceiverstation may include a signal shaper and transmitter module 717 whichreceives (for the call in question) either data 718 from a full ratechannel encoder 7131 or data 719 delivered by a selector 720 whichdelivers alternately data 72 produced by a channel coder 713N andasynchronous data 722.

The change mode commands and the quality information are transmitted inthe protocol data, for example. In the case of the GSM system, they canbe placed in the ACCH or FACCH channels.

The decision process of the module 71 will now be described withreference to FIG. 8, in the case of two coding modes (this is readilygeneralizable, for example from FIG. 10).

The module effects (81) a measurement 84 of quality in the downlinkdirection (C/I₋₋ dl) and transmits (82) it to the base transceiverstation. At the same time the base transceiver station effects (83) ameasurement (85) of quality in the uplink direction (C/I₋₋ up). Fourprocesses are considered, depending on which of the followingbidirectional configurations is the current configuration:

1: half rate in both transmission directions,

2: full rate in both transmission directions,

3: modified half rate in the downlink direction and full rate in theuplink direction,

4: modified half rate in the uplink direction and full rate in thedownlink direction.

Only the case corresponding to configuration 1, the most complex, isdescribed.

The following operations are effected:

if C/I₋₋ dl<S[1] and C/I₋₋ up<S[1] (85) then choose configuration 2(86); request necessary additional resource (87); if resource isallocated (88) then transmit (89) command to mobile station if not: nomodification (810); if not

if C/I₋₋ dl>S[1] and C/I₋₋ up<S[1] (811) then choose configuration 3(812); request necessary additional resource (813); if resource isallocated (814) then transmit (815) command to mobile station if not: nomodification (810); if not

if C/I₋₋ dl<S[1] and C/I₋₋ up>S[1] (816) then choose configuration 4(817); request necessary additional resource (818); if resource isallocated (819) then transmit (820) command to mobile station if not: nomodification (810); if not

if C/I₋₋ dl>S[1] and C/I₋₋ up>S[1] (821) then no modification (822).

If the initial configurations are 2, 3 or 4, the same bidirectionalconfiguration choices are made, but the same resource is retained ifconfiguration 1 is not chosen. If the latter is selected, the excessresource is released.

As already indicated, the invention also applies to the transmission ofspeech. As before, various situations can be encountered:

there is a series of transmission modes for which the user bit rate isvariable but the raw bit rate is constant; the same radio resource istherefore always allocated; the mode may always be the same in bothdirections or the modes may be different but correspond to the sameresource,

there can be a series of transmission modes with a fixed user bit ratein accordance with the principle already described for data (inparticular for half rate and overprotected half rate coding modes),

in the most general case, there may be a series of coding modes thatcorrespond to user and raw bit rates, and therefore to allocatedresources, that are variable; the curves showing the performance interms of MOS (Mean Opinion Score) of the various modes as a function ofthe signal to interference ratio (C/I) can be plotted, as shown in FIG.13; the curves 131i,j correspond to the codes i, j, where i relates tothe raw bit rate and i to the channel coding rate. The value of iincreases as the raw bit rate increases and the value of i increases asthe robustness decreases. The codes 133 use two time slots per frame andthe codes 134 use one time slot per frame.

In this latter case, there are two feasible mode change strategies:

Given the quality information, the mode chosen for each direction is theone that provides the best quality in terms of MOS without considerationas to the differences between the radio resources of the modes. Thebidirectional resource corresponding to the more resource hungry mode ofthe two directions is then allocated. In the FIG. 13 example, C3,1,C3,2, C1,2 would be used in succession for a C/I continuouslydeteriorating (certain modes are therefore not used). The mode changedecision thresholds correspond to the intersections 1321 through 1325between the curves corresponding to these various modes.

Given the quality information, the less resource hungry mode is chosenfor each direction, which assures the required quality and provides thebest quality relative to the other equally resource hungry modes. In theFIG. 13 example the modes C3,1, C2,1, C1,1, C3,2, C2,2, C1,2 would beused in succession for a C/I continuously deteriorating. This means thatas C/I deteriorates, the resource is changed only if there is no otherway to assure the required quality. If C/I improves, the mode is changedfor the less resource hungry mode to assure the required quality ofservice. The decision thresholds can correspond to the intersectionsbetween the curves of the modes used mentioned above.

In both cases n modes that can be renumbered Ci(i=1, . . . , n) and nquality thresholds Si(i=1, . . . , n) are used, which may be illustratedby a graph identical to that in FIG. 11.

The only difference compared to the case already dealt with is that twomodes with different suffixes do not systematically correspond to adifferent resource and this leads to a modified mode only if the raw bitrate is different.

The same type of algorithm can be used as described with reference toFIG. 10. Two series of thresholds can therefore be provided according towhether a change to a more robust or a less robust mode to avoid"ping-pong" effects is anticipated.

What is claimed is:
 1. A method of adapting the air interface in amobile radio system enabling exchange of data in the form ofbidirectional digital signals between at least one mobile station and atleast one base transceiver station, said method comprising the stepsof:providing the system with at least two coding modes, each modecorresponding to a predetermined source code and a predetermined channelcode for the transmission of a wanted signal for each transmissiondirection, and each source code corresponding to a given user bit rateand each channel code corresponding to a given coding efficiency andtherefore to a given total bit rate for a given source code; at the timeof a call between a mobile station and a base transceiver station,making two separate analyses of transmission quality for eachtransmission direction, respectively; for each transmission direction,selecting one of said coding modes in accordance with the correspondingtransmission quality analysis; and allocating a quantity of transmissionresource for each transmission direction in accordance with saidanalyses of transmission quality and the available transmissionresource; wherein transmission resource allocation is symmetrical foreach transmission direction, wherein more transmission resource isallocated if the coding mode selected in at least one transmissiondirection corresponds to a total bit rate incompatible with the resourceallocated in the current transmission mode and the additionaltransmission resource required is available, and wherein lesstransmission resource is allocated if the coding modes selected in thetwo transmission directions correspond to total bit rates compatiblewith said allocation.
 2. A method of adapting the air interface in amobile radio system enabling exchange of data in the form ofbidirectional digital signals between at least one mobile station and atleast one base transceiver station, said method comprising the stepsof:providing the system with at least two coding modes, each modecorresponding to a predetermined source code and a predetermined channelcode for the transmission of a wanted signal for each transmissiondirection, and each source code corresponding to a given user bit rateand each channel code corresponding to a given coding efficiency andtherefore to a given total bit rate for a given source code; at the timeof a call between a mobile station and a base transceiver station,making two separate analyses of transmission quality for eachtransmission direction, respectively; for each transmission direction,selecting one of said coding modes in accordance with the correspondingtransmission quality analysis; and allocating a quantity of transmissionresource for each transmission direction in accordance with saidanalyses of transmission quality and the available transmissionresource; wherein at least one modified transmission mode is defined inwhich the resource allocated to at least one transmission direction isgreater than the resource needed to transmit information coded in theselected coding mode, said coded information being divided between afraction of the time slots corresponding to said allocated resource. 3.The method claimed in claim 2 wherein the time slots not used by saidcall do not carry any signal.
 4. The method claimed in claim 2 whereinthe time slots not used by said call are assigned to the transmission ofasynchronous data signals.
 5. The method claimed in claim 2, whereinsaid system provides at least two transmission modes including:a firsttransmission mode corresponding to a first quality level whereby saiddata is transmitted at the rate of one time slot every signal frame inboth transmission directions, and a second transmission modecorresponding to a second quality level below said first quality levelwhereby said data is transmitted at the rate of one time slot every twosignal frames in both directions; andwherein said at least one modifiedtransmission mode consists of: transmitting information coded in a firstcoding mode in a first transmission direction at the rate of one timeslot every signal frame, and transmitting information coded in a secondcoding mode in a second transmission direction at the rate of one timeslot every two signal frames,the resource allocated to the call in eachof the two transmission directions corresponding to the resource neededto transmit data in the first coding mode.
 6. A method of adapting theair interface in a mobile radio system enabling exchange of data in theform of bidirectional digital signals between at least one mobilestation and at least one base transceiver station, said methodcomprising the steps of:providing the system with at least two codingmodes, each mode corresponding to a predetermined source code and apredetermined channel code for the transmission of a wanted signal foreach transmission direction, and each source code corresponding to agiven user bit rate and each channel code corresponding to a givencoding efficiency and therefore to a given total bit rate for a givensource code; at the time of a call between a mobile station and a basetransceiver station, making two separate analyses of transmissionquality for each transmission direction, respectively; and for eachtransmission direction, selecting one of said coding modes in accordancewith the corresponding transmission quality analysis; wherein thetransmission quality analysis consists in determining at least one ofthe following:the bit error rate of the received signal, the power ofthe received signal, the distance between the mobile station and thebase station, an estimate of the impulse response of the transmissionchannel, the time alignment, the signal to noise ratio, the signal tointerference ratio, the traffic load.
 7. A method of adapting the airinterface in a mobile radio system enabling exchange of data in the formof bidirectional digital signals between at least one mobile station andat least one base transceiver station, said method comprising the stepsof:providing the system with at least two coding modes, each modecorresponding to a predetermined source code and a predetermined channelcode for the transmission of a wanted signal for each transmissiondirection, and each source code corresponding to a given user bit rateand each channel code corresponding to a given coding efficiency andtherefore to a given total bit rate for a given source code; at the timeof a call between a mobile station and a base transceiver station,making two separate analyses of transmission quality for eachtransmission direction, respectively; and for each transmissiondirection, selecting one of said coding modes in accordance with thecorresponding transmission quality analysis; wherein the selection of acoding mode includes a step of comparing information representative oftransmission quality with at least one predetermined threshold; whereinsaid information representative of quality is compared to differentthresholds according to the required quality level for the call inprogress.
 8. The method claimed in claim 7 wherein said thresholds arepredetermined values of the signal to interference ratio.
 9. The methodclaimed in claim 8 wherein a system operability threshold is definedand:for at least a first higher quality type of service said qualityinformation is compared with a first threshold higher than saidoperability threshold, and for at least a second lower quality type ofservice said quality information is compared with a second thresholdlower than said operability threshold.
 10. A method of adapting the airinterface in a mobile radio system enabling exchange of data in the formof bidirectional digital signals between at least one mobile station andat least one base transceiver station, said method comprising the stepsof:providing the system with at least two coding modes, each modecorresponding to a predetermined source code and a predetermined channelcode for the transmission of a wanted signal for each transmissiondirection, and each source code corresponding to a given user bit rateand each channel code corresponding to a given coding efficiency andtherefore to a given total bit rate for a given source code; at the timeof a call between a mobile station and a base transceiver station,making two separate analyses of transmission quality for eachtransmission direction; respectively, and for each transmissiondirection, selecting one of said coding modes in accordance with thecorresponding transmission quality analysis; wherein the selection of acoding mode includes a step of comparing information representative oftransmission quality with at least one predetermined threshold, andwherein two sets each of at least one threshold are defined, the firstset being used if the measured transmission quality deteriorates and thesecond set being used if the measured transmission quality improves. 11.A method of adapting the air interface in a mobile radio system enablingexchange of data in the form of bidirectional digital signals between atleast one mobile station and at least one base transceiver station, saidmethod comprising the steps of:providing the system with at least twocoding modes, each mode corresponding to a predetermined source code anda predetermined channel code for the transmission of a wanted signal foreach transmission direction, and each source code corresponding to agiven user bit rate and each channel code corresponding to a givencoding efficiency and therefore to a given total bit rate for a givensource code; at the time of a call between a mobile station and a basetransceiver station, making two separate analyses of transmissionquality for each transmission direction, respectively; and for eachtransmission direction, selecting one of said coding modes in accordancewith the corresponding transmission quality analysis; and wherein thedecision to change coding mode and/or transmission mode is taken in saidbase transceiver station, said mobile station transmitting to said basetransceiver station information representative of transmission qualityin the base transceiver station to mobile station direction and saidbase transceiver station transmitting to said mobile station informationrepresentative of said decision.
 12. A method of transmitting modifieddata in a mobile radio system enabling bidirectional exchange of databetween at least one mobile station and at least one base transceiverstation, said method comprising the steps of:providing the system withat least two transmission modes, a first transmission mode correspondingto at least one first coding mode and necessitating a first allocationof resource in each transmission direction, and a second transmissionmode corresponding to at least one second coding mode and necessitatinga second allocation of resource, less than said first allocation ofresource, in each transmission direction; transmitting data coded in oneof said first coding modes in a first transmission direction;transmitting data coded in one of said second coding modes in a secondtransmission direction; causing the resource allocated to the call ineach transmission direction to correspond to the resource allocated inaccordance with said first transmission mode; and transmitting in thesecond transmission mode by using only a part of the availabletransmission resource.
 13. A base transceiver station of a mobile radiostation using a method of adapting the air interface in a mobile radiosystem enabling exchange of data in the form of bidirectional digitalsignals between at least one mobile station and at least one basetransceiver station, said method comprising the steps of: providing thesystem with at least two coding modes, each mode corresponding to apredetermined source code and a predetermined channel code for thetransmission of a wanted signal for each transmission direction, andeach source code corresponding to a given user bit rate and each channelcode corresponding to a given coding efficiency and therefore to a giventotal bit rate for a given source code; at the time of a call between amobile station and a base transceiver station, making two separateanalyses of transmission quality for each transmission direction,respectively; for each transmission direction, selecting one of saidcoding modes in accordance with the corresponding transmission qualityanalysis;said base transceiver station comprising:means for determiningat least a first indication representative of transmission quality inthe mobile station to base transceiver station direction, means forreceiving a second indication representative of transmission quality inthe base transceiver station to mobile station direction, means formodifying the coding mode and/or the transmission mode in eachtransmission direction in accordance with said first and secondindications, and means for transmitting to said mobile stationinformation representative of the coding and/or transmission modesselected.
 14. A mobile station of a mobile radio system implementing amethod of adapting the air interface in a mobile radio system enablingexchange of data in the form of bidirectional digital signals between atleast one mobile station and at least one base transceiver station, saidmethod comprising the steps of: providing the system with at least twocoding modes, each mode corresponding to a predetermined source code anda predetermined channel code for the transmission of a wanted signal foreach transmission direction, and each source code corresponding to agiven user bit rate and each channel code corresponding to a givencoding efficiency and therefore to a given total bit rate for a givensource code; at the time of a call between a mobile station and a basetransceiver station, making two separate analyses of transmissionquality for each transmission direction, respectively; for eachtransmission direction, selecting one of said coding modes in accordancewith the corresponding transmission quality analysis;said mobile stationcomprising:means for determining at least one indication representativeof transmission quality in the base transceiver station to mobilestation direction, means for transmitting said indication to said basetransceiver station, and means for receiving an indicationrepresentative of the coding and/or transmission modes selected.